TOKYO — North Korea claimed Thursday it destroyed its key nuclear weapons testing site, setting off explosions to collapse underground tunnels hours before President Trump called off a planned June 12 summit with North Korean leader Kim Jong Un.

The North had used the site, tucked into a remote, mountainous area, to detonate six increasingly large nuclear bombs over 11 years.

The apparent demolition was widely seen as a diplomatic gesture toward Washington, even as it remained unclear whether the made-for-TV blasts marked any significant change in the North’s nuclear capabilities.

Now, however, the timing of Trump’s cancellation is likely to add new tensions and uncertainty on the Korean Peninsula after groundbreaking overtures by the North with the United States and ally South Korea.

In calling off the summit with Kim in Singapore, Trump cited “open hostility” by North Korea in criticizing recent U.S. comments about efforts to dismantle the North’s nuclear weapons program.

The fact that Kim’s regime went ahead with the destruction of the Punggye-ri nuclear testing site appeared to signal it was still willing to embark on a diplomatic journey with the United States.

Trump’s statement in a letter to Kim left open the prospect the two could meet in the future but said a summit was “inappropriate, at this time.”

Early Friday, North Korean Vice Foreign Minister Kim Gye Gwan issued a conciliatory statement, saying the regime was ready to meet with the United States “at any time.”

Trump’s decision did not match with “the world’s desire,” Kim said in the statement published by the North’s official Korean Central News Agency. A summit is urgently needed to deal with the “grave hostilities” in the relationship between North Korea and the United States, the statement said.

“Leader Kim Jong Un had focused every effort on his meeting with President Trump,” the vice foreign minister said.

Journalists brought to the North’s testing site reported powerful blasts there. But the Kim regime did not allow any experts to observe the events, making it difficult to assess what exactly it had done. Most analysts remain highly doubtful North Korea is really prepared to give up its nuclear weapons program.

DO NOT USE THIS IMAGE. THIS IMAGE IS FOR ONE TIME USE WITH AMP-STORIES ONLY. WASHINGTON, DC - MAY 23: President Donald J. Trump stops to talk to reporters and members of the media as he walks from the Oval Office to board Marine One on the South Lawn of the White House on Wednesday, May 23, 2018 in Washington, DC. (Photo by Jabin Botsford/The Washington Post)

But at least to the untrained eye, the regime appeared to make good on its promise to close the testing site. Journalists were invited as witnesses from Britain, China, Russia, South Korea and the United States.

“There was a huge explosion; you could feel it,” said Tom Cheshire of Sky News, a British broadcaster, describing one of the detonations, which he witnessed from about 500 yards away. “Dust came at you, the heat came at you. It was extremely loud. It blew an observation tower to complete smithereens.”

The reporters were unable to immediately send images because of a lack of Internet or cell access.

Restrictions on the journalists were tight on their 300-mile journey — involving a 12-hour train ride and then four hours on a bus, followed by an hour hiking through the mountains.

North Korea’s Nuclear Weapons Institute declared Thursday night that the site was no longer usable.

“Dismantling the nuclear test ground was done in such a way as to make all the tunnels of the test ground collapse by explosion and completely close the tunnel entrances,” a deputy director at the institute said, according to state media.

No radiation leaked from the site, and the explosions did not cause any environmental damage, it said, statements that could not be independently verified. The journalists had their dosimeters confiscated so they could not check for radiation.

Since that test, there have been suggestions Mount Mantap might be suffering from “tired mountain syndrome,” and experts said the north portal tunnels had become unusable.

Those tunnels were destroyed with dynamite at about 11 a.m. local time, according to South Korean pool reports from the scene.

However, the west and south portals had never been used, and experts said they were still considered viable for future tests. The west portal was blown up shortly after 2 p.m., and the southern one at 4 p.m. Then barracks, observation towers and other buildings were destroyed, according to the reports.

The east portal, through which North Korea conducted its first nuclear test, in 2006, has been abandoned for more than a decade and is no longer accessible.

There remains a considerable amount of skepticism about Thursday’s events, given that North Korea invited foreign media to film the spectacular destruction of the cooling tower at the Yongbyon nuclear plant in 2008, part of a denuclearization deal that was meant to cut off North Korea’s access to plutonium.

The images looked impressive at the time — then it transpired a couple of years later that North Korea had been building a huge uranium-enrichment facility all the while, giving it another source of fissile material.

Still, analysts said Thursday’s demolition was a move in the right direction, even if it was little more than a gesture.

“This will be highly symbolic and a diplomatic first step,” said Frank Pabian, a former nuclear nonproliferation and satellite imagery expert at the Los Alamos National Laboratory. “But in and of itself, it won’t change anything about North Korea’s nuclear capabilities.”

Although no nuclear experts were allowed to attend the event, Pabian, who now writes for the specialist website 38 North, said officials from organizations such as the Comprehensive Test Ban Treaty Organization would still be able to carry out testing if they were ever granted access to the site.

North Korea has signaled it no longer needs to test its nuclear devices because it has mastered the technology, a claim that is not without credibility given its quantifiable advances over 11 years of testing.

“The mission of the northern nuclear test ground has thus come to an end,” Kim told a Workers’ Party meeting in Pyongyang, the North Korean capital, last month. By acquiring this “powerful treasured sword for defending peace,” North Koreans could now “enjoy the most dignified and happiest life in the world,” he said.

North Korea’s Foreign Ministry confirmed May 12 that the site would be “completely closed.”

This came after a historic summit with South Korean President Moon Jae-in late last month, when Kim agreed to embark on a number of steps to show he was serious about dealing with the United States, the North’s avowed enemy.

He also mentioned a plan to work toward “the complete denuclearization of the Korean Peninsula” — a phrase Trump took to mean Kim wanted to give up his weapons, while most analysts said it was code for a drawn-out process under which both sides would have to make concessions.

In an interview broadcast Thursday before officially canceling the summit, Trump said the United States might accept a “phase-in” of North Korea’s denuclearization, but he insisted the dismantling of the nuclear program must progress “rapidly.”

In the last few days North Korea had taken to caustic denunciations of American declarations.

The two sides appeared to be using a “willingness to walk” from the summit as a source of leverage, said Katrin Katz, a North Korea expert at Northwestern University.

“Thanks to the North Koreans’ rhetorical pivot, both sides are now in a competition to seem the least desperate to meet — and to place the blame for not meeting on the other side,” she said before Trump announced the cancellation, citing the harsh North Korean rhetoric.

“This type of behavior contrasts with North Korea’s recent charm offensive but not its longer-term behavior,” she said, noting it could have a positive effect: It could give an American president who likes to “wing it” more time to come up with a careful plan.